In order to increase power output per displaced volume of an internal combustion engine, one can increase the density of the air at the entry of the engine using a boost compressor. Since there is a limitation to the peak pressure that the engine can tolerate, it is common practice to decrease the volumetric compression ratio of the engine when high boost pressure is used. However the boost pressure that can be provided by the compressor is limited during start-up conditions, and the engine has to rely mainly on its own compression to provide sufficient pressure to light the fuel-air mixture; a small volumetric compression ratio thus impede the performance of the engine assembly at start-up. The volumetric compression ratio must accordingly be selected to accommodate the conflicting requirements of limiting the peak pressure while providing sufficient pressure during start-up.